A grain oriented electrical steel sheet is mainly utilized as an iron core of a transformer and required to exhibit superior magnetization characteristics, e.g. low iron loss in particular.
In this regard, it is important to highly align or accumulate secondary recrystallized grains of a steel sheet in (110)[001] orientation, i.e. what is called “Goss orientation”, and reduce impurities in a product steel sheet. However, there are restrictions on controlling crystal grains and reducing impurities in view of production cost. Accordingly, there has been developed a technique of introducing non-uniformity into a surface of a steel sheet by physical means to subdivide the width of a magnetic domain to reduce iron loss, i.e. magnetic domain refinement technique.
For example, JP-B 57-002252 proposes a technique of irradiating a steel sheet as a finished product with a laser to introduce high-dislocation density regions into a surface layer of the steel sheet, thereby narrowing magnetic domain widths and reducing iron loss of the steel sheet. The magnetic domain refinement technique using laser irradiation was improved thereafter (see JP-A 2006-117964, JP-A 10-204533, JP-A 11-279645 and the like), so that a grain oriented electrical steel sheet having good iron loss properties can be obtained.
Further, JP-B 06-072266 proposes a technique of controlling magnetic domain widths by irradiation of electron beam.
However, there is a problem in that, when such various types of grain oriented electrical steel sheets as described above subjected to magnetic domain refinement of irradiating the steel sheets with a laser or an electron beam are each assembled into real transformer devices, these transformers make louder noises than grain oriented electrical steel sheets not subjected to magnetic domain refinement. Further, there arises another problem in that iron loss of a real transformer device hardly improves after all, although iron loss of a grain oriented electrical steel sheet as an iron core material has been reduced by magnetic domain refinement through irradiation by laser or electron beam, i.e. a problem of extremely poor “building factor” (BF).
It could therefore be helpful to provide a grain oriented electrical steel sheet which, when assembled into a real transformer device, exhibits excellent low-noise properties and low-iron loss properties, and an advantageous method for manufacturing the grain oriented electrical steel sheet.